The present invention relates to microporous polyolefinic hollow fibers having improved characteristics, and to a process and apparatus for preparing such fibers. It particularly relates to improvements in the spinning of hollow polyolefinic precursor fibers which yield microporous hollow fibers having improved structural and performance characteristics.
Hollow porous fibers are well known in the art. See, for example, U.S. Pat. Nos. 4,020,230; 4,082,658, and 4,214,020, as well as European Published Patent Application No. 0 005 866, published Dec. 12, 1979. The advantages of permeable hollow fibers over permeable films are also well known. For example, porous hollow fibers possess a larger surface area per unit volume than a flat film of similar porous configuration. Consequently, there has been a growing tendancy to employ, if possible, hollow microporous fibers in those applications typically reserved for permeable films.
One particular important use for hollow microporous fibers is as a blood oxygenator as illustrated by U.S. Pat. No. 4,020,230 which discloses hollow microporous fibers prepared from polyethylene. As is well known the properties required in a blood oxygenator membrane include good gas permeability with respect to gaseous oxygen and carbon dioxide, chemical stability, blood compatibility or substantially non-thrombogenic behavior in blood containing environments, sufficiently hydrophobic character to serve as a water vapor barrier, ease in manufacture, nontoxicity, relative inertness to body fluids, and mechanical strength and handling properties adequate for facilitating the assembly and use of blood oxygenation devices.
Microporous polypropylene films have previously been employed as blood oxygenation membranes and such films have been found to meet all of the above requirements. However, because of the relatively low surface area of such films, relatively large volumes of blood must be removed from the body to achieve the required oxygen and carbon dioxide gas transfer. In contrast, hollow polypropylene microporous fibers offer the advantage of being able to achieve the same gas transfer using much lower volumes of blood.
According to one known technique for preparing hollow polyolefinic microporous fibers, hollow precursor fibers are melt spun, annealed, stretched to open up the crystalline structure of the polymer and to impart microporosity thereto, and then heat set to stabilize the resulting microporous structure. Commonly assigned U.S. patent application Ser. No. 11,740, filed Feb. 13, 1979 in the names of Howard J. Davis, Willard c. Brinegar, Phillip L. Scoville and Peter K. Sullivan, entitled "Process for Preparing Hollow Microporous Polypropylene Fibers", for example, discloses a process for preparing hollow open-celled polypropylene microporous fibers having an oxygen flux of at least 35 cc/cm.sup.2. min. at 10 psi which comprises (a) melt spinning a hollow non-porous polypropylene precursor fiber having an average inner diameter of at least 140 microns, an average inner diameter to average wall thickness ratio of from about 8:1 to about 40:1, an elastic recovery from 50% extension at 25.degree. C., 65% relative humidity and zero recovery time of at least 50% and degree of orientation as determined from the half width of the wide angle (110) X-ray diffraction arc of not greater than 25.degree. from isotatic polypropylene having a melt index of at least 1 at a spinning temperature of at least 230.degree. C. and a drawdown ratio of at least about 40; (b) annealing the precursor fibers at a temperature between about 50.degree. C. and 165.degree. C. for a period from about 0.5 second to about 24 hours; (c) cold stretching the non-porous annealed precursor fibers at a temperature of less than about 100.degree. C. followed by hot stretching the precursor fibers at a temperature above 100.degree. C. with a total degree of combined stretching of from 80 to about 200%; and (d) heat setting the resulting stretched fibers under tension to produce open celled hollow microporous polypropylene fibers having an average inner diameter of at least 100 microns.
U.S. Pat. No. 4,055,696 describes a somewhat similar type of process for the preparation of microporous polypropylene hollow fibers wherein a cold stretching technique is employed to prepare the hollow polypropylene microporous fibers. This process requires that the size of the pores be kept within a specified range by limiting the degree and temperature of cold stretch to 30 to 200% of the original fiber length and less than 110.degree. C., respectively. The resulting cold stretched fibers which have been previously annealed are heat set at a temperature at or above the initial annealing temperature, employed prior to stretching as described above. Annealed, cold stretched, heat set, hollow fibers prepared in accordance with this patent tend to exhibit varying degrees of shrinkage depending on the relationship of the prior annealing temperature and duration to the heat setting temperature and duration.
Japanese Kokai Pat. No. Sho 53 [1978] -38715 published Apr. 10, 1978 is directed to an improvement in the method for preparing porous polypropylene hollow fibers disclosed in U.S. Pat. No. 4,055,696. The improvement comprises controlling the annealing temperature to be below 155.degree. C. and controlling the heat setting temperature after cold stretching to be from 155.degree. to 175.degree. C. for from 3 seconds to 30 minutes.
Further improvements in the basic process of U.S. Pat. No. 4,055,696 are described in Japanese Kokai Pat. Nos. Sho 54 [1979] - 34418 and 68414. In the first of these publications, a hollow precursor fiber having a elastic recovery of at least 60% is stretched 30-400% and then heat set under conditions such that the fiber relaxes from 5-95% of its stretched length. In the second of these publications, the hollow precursor fiber is subjected to a cold-stretch, hot stretch stretching sequence and then heat set at a temperature of from 130.degree.-165.degree. C.
In the melt spinning of precursor for microporous hollow fibers, special characteristics must be imparted to the fiber. The most important are:
(a) mechanical strength; PA0 (b) uniform geometric shape; PA0 (c) high molecular orientation to allow the generation of porosity in subsequent stretching operations; and PA0 (d) uniform molecular orientation throughout the hollow fiber structure so that uniform porosity results.
The traditional approach to the melt extrusion of polymeric hollow precursor fibers requires a source of molten, pressurized polymer, a die to form the hollow precursor fiber, a method of cooling the hollow precursor to a non-sticking solid, and a winding device. The usual arrangement of these elements is to have the molten polymer enter the hollow fiber-forming capillaries of the die from above. The molten fibers flow downward from the die, through a zone where cooling takes place, to the wind-up device.
Conventionally, the hollow precursor fiber is cooled (quenched) by passing an air stream across the hollow precursor fiber. In this type of cooling arrangement, any single filament sees only a cross flow of cooling air, i.e. the local velocity vector of the air impinges on only one side of the filament. Different portions of the hollow precursor fiber thus experience different degrees of quenching, resulting in variations in molecular orientation throughout the fiber, and ultimately in variations in porosity throughout the microporous hollow fiber as well as variations in the rate of cooling throughout the fiber leading to microporous hollow fibers with substantial variations in inner diameter and wall thickness.
Processing of polyolefins into high quality microporous hollow fibers has heretofore also been limited by the non-Newtonian nature of such polymers. As polyolefins such as polypropylene exit the die capillary, the melt expands in a phenomenon commonly referred to as die swell. Through a complex interaction of apparent extrusion viscosity, cooling rate at quench and applied stress, die swell makes spinning of a uniform microporous hollow fiber (MHF) precursor difficult in the traditional down spinning configuration. In addition, die swell tends to retard the drawdown of the hot extruded fiber and thereby to decrease the amount of stress applied to the hot precursor and at the same time the rate of cooling of the hot precursor. Since the amount of molecular orientation present in the precursor fiber is directly related to the amount of stress applied to the fiber as it exits the spinneret and the quench rate, die swell limits the amount of molecular orientation which can be imparted to the hot fiber by the traditional down spinning approach.
Since the degree of die swell increases with the molecular weight of the polyolefin, one approach to mitigating the problems associated with die swell has been to use lower molecular weight polymer (melt index greater than about 8 for polypropylene) and wind up the spun fiber at higher speeds (400-600 m/min). The lower M.W. resin exhibits lower viscosity exiting the spinneret. Die swell is reduced and the fiber draws down close to the spinneret. This allows for more rapid cooling because of the higher surfce area and linear speed of the filament. The shortcomings of this process are in the inflexibility of resin choice and product porosity characteristics. Higher molecular weight resins are generally preferred since they result in stronger microporous hollow fiber with better resistance to degradation. In addition, the molecular weight of the resin has a strong influence on the size of the pores which form on stretching.
It is also known in the art that microporous polyolefinic films may be prepared by melt extruding a precursor film by the so-called blown film technique wherein a polyolefinic precursor film is extruded in the form of a hot tube which is expanded by internal air pressure as it exits the die. In Example IX of U.S. Pat. No. 3,558,764, for example, a polyethylene precursor film is melt extruded at 380.degree. F. in the form of a hot tube, expanded 1.5 times in diameter by internal air pressure and then cooled using an air ring located above the extrusion die. In this regard, see also U.S. Pat. Nos. 3,679,538; 3,801,404; 3,801,692; and 3,839,240. Extrusion of precursor films by the blown film technique is not analogous, however, to the spinning of hollow precursor fibers. With the blown film technique, die swell does not effect the blowing of the hot film and is thus not a problem. In contrast, in the spinning of hollow precursor fibers, the inner diameter of the hollow precursor fiber decreases relative to its extruded size. Die swell retards this desired reduction in fiber inner diameter resulting in the disadvantages noted above.
It would thus be desirable to provide a new method for spinning polyolefinic precursor fibers which overcomes the disadvantages of the prior art and which results in novel microporous hollow fibers having a unique combination of physical characteristics and performance.